1. Field of the Invention
This invention relates to a method and apparatus for control of tachycardia, particularly ventricular tachycardias.
2. Description of the Prior Art
Tachycardia is a condition in which the atria, ventricles or both chambers of the heart beats very rapidly, and not within the normal physiological range, typically exceeding 160 occurrences per minutes. Atrial tachycardia is the medical term assigned to the condition in which rapid and regular succession of P waves of the PQRST waveform complex occur. The rate of occurrence is in excess of the physiological range normally encountered in the particular patient.
Paroxysmal supra-ventricular tachycardia is the medical term assigned to the condition in which there is a sudden attack of rapid heart action in the atria or in the atrial-ventricular node. The main characteristics are the same as those in atrial tachycardia.
Normally atrial tachycardia and paroxysmal supra-ventricular tachycardia is not a life threatening condition, unless it progresses into ventricular tachycardia or fibrillation. Ventricular tachycardia is the medical term assigned to the condition in which rapid and regular succession of R waves of the PQRST waveform complex occur. Again, the rate of occurrence is in excess of the physiological range of the particular patient and can, if left untreated, progress into ventricular fibrillation. In ventricular fibrillation, the ventricles are unable to pump blood in a coordinated fashion and heart output drops to a level dangerous to the patient.
Typically, these life-threatening conditions of ventricular tachycardia or ventricular fibrillation must be treated by drug therapy or by electrical stimulation, such as cardioversion or defibrillation, and the patient is confined for some time in the hospital.
In the past 15 years, attention has been paid to the use of lower energy stimulation to treat each of the above mentioned tachycardia conditions. Initially, investigators employed asynchronous pacemakers which were triggered typically by the patient into asynchronous pacing at a low rate, for underdrive pacing, or at a high rate, for overdrive pacing. Such pacing was accomplished in the hospital setting by a temporary pacemaker and catheter operated by the physician or permanent pacemakers and leads were implanted in the patient which could be activated by a magnet applied over the pacemaker by the patient when the patient felt a tachycardia condition. In addition, certain pacemakers were devised to detect a tachycardia condition and deliver a stimulus to either cardiovert, underdrive or overdrive the heart (see for example Gemmer, U.S. Pat. No. 3,460,542 and Zacouto U.S. Pat. No. 3,857,399).
In the Berkovits U.S. Pat. No. 3,698,398, one of the co-inventors of this invention proposed that atrial tachycardia and paroxysmal supra-ventricular tachycardias be treated by a pacemaker, either implantable or external, which when actuated by a sensed tachycardia condition, would deliver a burst of stimuli to the atria. That invention further contemplated that the interval between successive stimuli would either be increased or decreased in a progressive manner so that one of the intervals in the succession of intervals would be the critical interval that it would allow interruption of the tachycardia mechanism. That modality has been called rate scanning or entrainment of the tachycardia interval.
More recently, rate scanning has been implemented in microcomputer based tachyarrhythmia termination systems, such as the Medtronic Model SPO 500 antitachyarrhythmia system. Another system which has been recorded in the article entitled, "A Microcomputer-Based Tachycardia Termination System--A Preliminary Report", by J. Camm et al in the Journal of Medical Engineering and Technology, Vol. 4, No. 2, March 1980, the authors' report on their experience with a rate scanning regimen which was programmed through a microcomputer system. Very generally, the authors define their system as auto-decremental pacing which involves:
1. Sensing and recognition of tachycardia, defined for this specific purpose as four consecutive P-P intervals between 218 and 375 ms.
2. Pacing on an initial discharge interval equal to the P-P interval-tachycardia cycle length (TCL) minus a decrement value (D). The discharge interval of the pacemaker pace and cycle length (PCL) progressively shortens as with each subsequent beat an additional decrement is subtracted. The minimum PCL of the pacemaker is 280 ms.
3. The pacing sequence continues until the next discharge interval would prolong the total duration of pacing beyond a preset period (P). P and D are operator controlled variables.
The pacemaker hardware employed by the authors consisted of a microcomputer unit based on a Signetics 2650 eight bit microprocessor which controls an isolated stimulation module. The input to the microcomputer was derived from the threshold detector output of an isolation amplifier. Both the stimulator and receiver module were part of a device Neurolog 4279 arrhythmia investigation system. A Hewlett-Packard 9830 desktop calculator was programmed to simulate the performance of the auto decremental pacing system. The autodecremental modality for tachycardia termination was assessed and selected patients with paroxysmal supra-ventricular tachycardia. Two bipolar electrocatheters were introduced percutaneously for sensing and pacing purposes. Autodecremental atrial pacing with P values between 2 and 5 seconds and D values between 3 and 20 ms terminated all tachycardias without initiating other atrial tachyarrhythmias.
The authors of this article go on to state that this system had proved successful in the conversion to sinus rhythm of a variety of supra-ventricular tachycardias.
The literature generally suggests that overdrive burst stimulation or rate scanning stimulation may he used for either atrial or ventricular tachyarrhythmia conditions. However, actual reported experience of use of rate scanning or burst overdrive stimulation to treat ventricular tachycardia successfully is scant indeed. Most investigators have confined their efforts to the treatment of atrial or supra-ventricular tachycardias due to the danger involved in stimulating into the vulnerable zone of the ventricular tachycardia cycle and initiating fibrillation. The present invention overcomes that danger through the relatively straightforward technique of sensing natural depolarizations of the ventricle during the rate scanning interval and resetting the rate scanning time interval in response thereto to thereby avoid competitive pacing.